It actually makes so much sense why Guts had that flashback now. Thanks to his traumatic past, all sex ever was to him previously was an act of dominance and power, maybe even a little bit of lust, but never an act of love. With how he was man-handling Casca in the previous chapter, there's no doubt Guts was remembering how it played out last time he had that kind of experience. Instead it really came down to Casca to finally show Guts the side of sex that can actually be considered love-making and not just "fucking" for a lack of a better term. Thanks to Casca, Guts can now finally create a positive memory of the act to help erase or cancel-out some of the trauma from his past. What a woman!
I don't think I've ever seen an anime or read a manga that handled a sex scene this maturely before, but it's just perfect!